Re: [PATCH v13 00/10] powerpc: Switch to CONFIG_THREAD_INFO_IN_TASK

[Christophe Leroy]

On 01/24/2019 01:06 AM, Michael Ellerman wrote:
> Christophe Leroy <christophe.leroy@xxxxxx> writes:
> > Le 12/01/2019 Ã 10:55, Christophe Leroy a ÃcritÂ:
> > > The purpose of this serie is to activate CONFIG_THREAD_INFO_IN_TASK which
> > > moves the thread_info into task_struct.
> > >
> > > Moving thread_info into task_struct has the following advantages:
> > > - It protects thread_info from corruption in the case of stack
> > > overflows.
> > > - Its address is harder to determine if stack addresses are
> > > leaked, making a number of attacks more difficult.
> >
> > I ran null_syscall and context_switch benchmark selftests and the result
> > is surprising. There is slight degradation in context_switch and a
> > significant one on null_syscall:
> >
> > Without the serie:
> >
> > ~# chrt -f 98 ./context_switch --no-altivec --no-vector --no-fp
> > 55542
> > 55562
> > 55564
> > 55562
> > 55568
> > ...
> >
> > ~# ./null_syscall
> >      2546.71 ns     336.17 cycles
> >
> >
> > With the serie:
> >
> > ~# chrt -f 98 ./context_switch --no-altivec --no-vector --no-fp
> > 55138
> > 55142
> > 55152
> > 55144
> > 55142
> >
> > ~# ./null_syscall
> >      3479.54 ns     459.30 cycles
> >
> > So 0,8% less context switches per second and 37% more time for one syscall ?
> >
> > Any idea ?
>
> What platform is that on?

It is on the 8xx

> On 64-bit we have to turn one mtmsrd into two and that's obviously a
> slow down. But I don't see that you've done anything similar in 32-bit
> code.
>
> I assume it's patch 8 that causes the slow down?

I have not digged into it yet, but why patch 8 ?


I run null_syscall with perf, and I get the following. Can we conclude 
on something with that ?

Without the serie:

# Overhead       Samples  Command       Shared Object      Symbol 

# ........  ............  ............  ................. 
........................................
#
    32.95%         46375  null_syscall  [kernel.kallsyms]  [k] DoSyscall
    23.64%         33275  null_syscall  [kernel.kallsyms]  [k] 
__task_pid_nr_ns
    15.47%         21778  null_syscall  libc-2.23.so       [.] 
__GI___getppid
     8.92%         12556  null_syscall  [kernel.kallsyms]  [k] 
__rcu_read_unlock
     5.69%          8014  null_syscall  [kernel.kallsyms]  [k] sys_getppid
     4.01%          5643  null_syscall  [kernel.kallsyms]  [k] 
__rcu_read_lock
     3.67%          5166  null_syscall  [kernel.kallsyms]  [k] 
syscall_dotrace_cont
     2.52%          3542  null_syscall  null_syscall       [.] main

With the serie:

    30.04%         56337  null_syscall  [kernel.kallsyms]  [k] DoSyscall
    13.89%         26060  null_syscall  [kernel.kallsyms]  [k] 
__rcu_read_unlock
    13.36%         25062  null_syscall  libc-2.23.so       [.] 
__GI___getppid
    12.73%         23872  null_syscall  [kernel.kallsyms]  [k] 
__task_pid_nr_ns
    11.21%         21033  null_syscall  [kernel.kallsyms]  [k] sys_getppid
     8.24%         15457  null_syscall  [kernel.kallsyms]  [k] 
syscall_dotrace_cont
     4.38%          8217  null_syscall  [kernel.kallsyms]  [k] 
ret_from_syscall
     2.54%          4773  null_syscall  null_syscall       [.] main


Christophe